Recovery of oil with viscous propane

ABSTRACT

In a method for recovering petroleum from earth formations, the petroleum is driven through the formation by two banks of material followed by gas. The first bank is a solution of a solid polymer in a mixture of propane and a higher molecular-weight hydrocarbon. The second bank is an enriched gas miscible with the liquid components of the polymer solution.

2,341,500 2/1944 Detling unnea States Patent Dwight L. Dauben;

[72] Inventors John C. Reed; Jack L. Shelton; Lyman 2,867,277 1/1959 Weinaug et a1. 166/273 2,880,801 4/1959 Crump 166/274X 3,003,554 10/1961 Craig, Jr., et al.... 166/274 3,074,481 1/1963 Habermann 166/274X 3,167,118 1/1965 l-labermann 166/273X 3,256,933 6/1966 Murphree et a1. 166/273X 3,354,953 11/1967 Morse 166/274X Primary Examiner-Stephen J. Novosad Attorneys Paul F. Hawley and Buell B. Hamilton ABSTRACT: In a method for recovering petroleum from earth formations, the petroleum is driven through the formation by two banks of material followed by gas. The first bank is a solution of a solid polymer in a mixture of propane and a higher molecular-weight hydrocarbon. The second bank is an enriched gas miscible with the liquid components of the polymer solution.

ZONE

GAS-ENRICHED GAS TRANSITION ENRICHED GAS POLYMER SOLUTION ENRICHED GAS TRANSITION ZONE POLYMER SOLUTION ZONE POLYMER SOLUTION OIL TRANSITION PATENIEDHARIBIBII 3.570.601

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AUBEN EED JACK L. SHELTON LYMAN YARBOROUGH Y INVENTORS ATTORNEY RECOVERY OF on. WITH VHSCGUS PRQPANE Many processes have been proposed for displacing crude oil from an oil-bearing earth formation toward a producing well through which the crude oil can be removed to the surface of the earth. One such process is described in.U.S. Pat. No. 3,354,953,Morse. in this process, a solvent, such as propane, with which the crude oil is miscible, is used to displace the crude oil through the formation. This solvent is, in turn, displaced through the formation by a gas miscible with the solvent. Morse points out that it is desirable to control the viscosity of the solvent to reduce the length of the transition zone between the crude oil and solvent. This can be done, for example, by mixing propane with something like kerosene, which is more viscous. Morse also suggests that the gas, which displaces the solvent, may be an enriched gas. For example, it may be methane enriched with higher hydrocarbons, such as propane. This enriched gas is then followed by dry gas. it kerosene is added to the propane, as suggested by Morse, to increase the propane viscosity, the volume of kerosene can easily become greater than the volume of propane. The amount of propane in the enriched gas must then be very high if a miscible drive is to be obtained. Unless a miscible drive is obtained, the bank of propane and kerosene is rapidly lost in the liquid phase which is left behind'in the formation.

In the process described in U.S. Pat. No. 3,330,345 Henderson et al., polymers are used in the solvent to increase the viscosity into an optimum range. The thickened solvent is then displaced, according to Henderson, et al. by an alcohol, which is, in turn, displaced by water. When use of polymers in the solvent is attempted to increase the solvent viscosity in a gasdrive process, however, several problems occur. if the polymer is a liquid, the same problems are present as with kerosene except that they are greatly magnified by the higher molecular weight of the polymer. if the polymer is a solid, little, if any, will dissolve in propane, so the desired viscosity cannot be reached.

An object of this invention is to provide a miscible-drive process in which gas is the final driving fluid and in which an optimum viscosity is obtained in the driving bank of propane in contact with the oil, this process using less heavy hydrocarbons, such as kerosene, in this bank and using less enriching material in an enriched gas bank which follows the oil-displacing bank. Other objects will be apparent from the following description and claims.

SUMMARY In general, we accomplish the objects of our invention by dissolving a solid polymer in a heavier hydrocarbon, such as heptane, to form a first solution, and then diluting this first solution by propane to form the oil-driving bank having an optimum viscosity. The presence of the polymer decreases the amount of heavier hydrocarbon used and thus decreases the amount of propane required in the enriched gas. Some of the polymer is usually precipitated in the transition zone between the propane bank and the enriched gas. The volume of precipitated polymer is very small, however, so little plugging occurs. The polymer precipitate is solid so no loss of the liquid bank occurs. The polymer is not required in the propane-gas transition zone, so no harm is done. No polymer is lost in the oil-propane transition zone where it is needed. Therefore, use of both a solid polymer and a heavier hydrocarbon in the propane bank achieves the desired viscosity with much more propane in the oil-displacing bank and much less propane in the enriched gas. This is a considerable advantage since propane costs roughly half as much as kerosene per unit of volume and natural gas costs only about one-tenth to one-fifteenth as much as propane.

This process employs a polymer solution, an enriched gas and a dry gas. The dry gas is almost always methane. The enriched gas is usually a mixture of methane and some commer cial LPG (liquefied petroleum gases). The LPG used in our process is mostly propane. The polymer solution is usually predominantly LPG with a smaller amount of a heavier hydrocarbon, such as heptane, and a polymer, such as polyisobutylene. All these materials have alternates which will be described later. For purposes of simplicity in the following description, however, the dry gas will usually be called methane, the enriching material in the enriched gas, and the principal ingredient of the polymer solution will be called propane, the heavier hydrocarbon will be called hcptane, and the polymer will be simply called a polymer.

in the drawings, FIG. 1 is a cross-sectional view of an oilbearing formation showing the compositions and sequence of zones moving through the formation.

FIG. 2 is a ternary diagram showing whether certain preferred compositions are single-phase or two-phase under various conditions.

Referring to the drawing in more detail, in FIG. 1, an injec tion well 1 l and a producing well 12 penetrate oil-bearing formation l3. Two batches of fluid 14 and 15 have been injected through well ill into the formation and have been driven to the position shown by driving gas 16. Batch 14 is preferably a solution of polyisobutylene in a mixture of propane and a light condensate having roughly the properties of heptane. Batch i5 is preferably a mixture of propane and a natural gas, the natural gas being almost entirely methane. The driving gas is preferably also a natural methane gas containing only small amounts of other gases.

FIG. 2 shows the phase behavior at 100 F. of a ternary system made up of methane, propane, and heptane. Suppose polymer solution M in FIG. 1 contains 25 percent heptane and 75 percent propane, exclusive of the polymer. This liquid is point A in FTG. 2. in order for the' enriched gas to miscibly displace this mixture in a reservoir at 100 F., the propane content of the enriched gas at pressures of 1,000, 1,500, 2,000, and 2,500 pounds per square inch absolute are shown at points B, C, D, and E, respectively. If the polymer solution contains percent propane, as at point F in the diagram, or contains 50 percentpropane, as at point G in the diagram, the corresponding enriched gas composition at 1,500 pounds per square inch absolute is shown at points it and J in the diagram. A propane concentration somewhat greater than indicated at points B, C, D, and the like, may be advisable to insure the existence of a miscible system. From this FIG. it will be obvious that it is best to use as much propane as possible in the polymer solution. This not only reduces the cost of the polymer solution, but also reduces the cost of the enriched gas. The principal limitation on the amount of propane in the polymer solvent is imposed by the polymer solubility.

it will be apparent that in designing a process for any particular oil under a given set of reservoir conditions, three steps are necessary. First, the optimum viscosity of the polymer solution must be determined. Next, a polymer solution having -this viscosity at reservoir temperature must be prepared using as little polymer and as much propane as possible. Third, the composition of an enriched gas miscible with the polymer solution and containing as much methane as possible must b determined.

The optimum propane viscosity for use in our process in any reservoir may be determined by flow tests somewhat described in U.S. Pat. No. 3,330,345 Henderson et al. except using a gas miscible with the oil-displacing solution as the final driving phase instead of the alcohol and water used in the Henderson et al. process. Such a test, however, does not take one factor into account. This is the tendency of the higher viscosity solution to force a displacing bank to flow through a larger percentage of the oil-bearing formation in its travel from the input to the output well. When this factor is considered, the actual optimum viscosity is somewhat higher than that determined by a simple linear flow test. ln general, the viscosity of the polymer solution in our process should preferably be between about 0.1 and about 0.2 times the viscosity of the displaced oil under reservoir conditions. Some When preparing a polymer solution, it should be realized that the solution will be prepared and pumped at the surface. Since polymer solubility will almost always be less at the surface than at reservoir conditions, it is polymer solubility at surface temperatures which is usually controlling.

in checking a particular polymer, a saturated solution in the polymer solvent, such as heptane, should first be prepared. The viscosity of this solution at reservoir temperature should then be measured. if the viscosity is not at least about 0.1 times the viscosity of the oil to be displaced, another polymer should, of course, be used. When a high-viscosity polymer solution in the heptane has been found, a small volume of this can be diluted with propane to be used in the process. At some point in the dilution, a precipitate will probably form. At a propane dilution slightly less than this, or at a slightly smaller polymer concentration in the original heptane solution, a clear solution will be formed. The viscosity of this clear solution at reservoir conditions can also be measured. From the two viscosity values, a close estimate can be made of the polymer concentration in the mixed propane and heptane to provide the desired viscosity. The concentration of polymer in the saturated mixture of propane and heptane also permits a close estimate of the maximum propane content of solvents for the desired concentration of polymer. This estimated composition can then be prepared and its viscosity at reservoir temperature can be measured. Minor adjustments can then be made, if necessary or desired, to obtain the exact viscosity required and the maximum propane content of the mixed polymer solvent.

Several polymers may be checked in the way described so the most economical system can be designed. Included in the cost of the system, of course, must be the cost of the enriched gas. This will usually be different for each polymer-solution composition.

The leanest enriched gas which is suitable can generally be determined in an equilibrium cell. The type of enriched gas drive where the enriching material in the gas dissolves in the displaced liquid until the liquid and gas become miscible usually does not occur in our process. if this type of system does occur, as described in more detail in U.S. Pat. No. 2,880,801, Crump, then the leanest enriched gas can best be determined by a flow test in which a transparent outlet pressure cell is provided to determine whether oneor two-phase displacement is occurring.

in an equilibrium cell, it will be best to avoid possible confusion by omitting the polymer from the liquid placed in the cell. As previously described, when the polymer solution becomes diluted by the enriched gas, a small amount of the polymer will generally precipitate and produce a cloudy solution.

As previously noted, several alternates exist for the polymer, heptane, propane, and. gas. A wide variety of polymers can be used. Examples include the esters and amides listed in U.S. Pat. No. 3,330,345 Henderson et al. A much more satisfactory class of polymers is made up of the hydrocarbon polymers. These include polyisoprenes, butyl rubber, styrene-butadiene copolymers, polystyrenes, mixed polybutylenes, and the like. The preferred polymer is polyisobutylene.

When considering alternates to heptane, volatility is important. A volatile solvent has good phase relationships with propane and methane. A volatile solvent, however, is usually a poor polymer solvent. A compromise must be made between the two requirements. i-lexane has better phase relationships than heptane but is not as good a polymer solvent. Octane is a better polymer solvent than heptane but has worse phase relationships. Actually, the two effects roughly ofiset each other for hexane and octane, so a mixture of hydrocarbons, having average molecular weight about like heptane, behaves in our process about the same as heptane. Aromatic hydrocarbons are very desirable. lieptane and benzene have similar phase characteristics and mutual solvent abilities for the propane and polymer. The aromatics are somewhat better for preventing asphalt precipitates in the formation. A light condensate from a high-pressure well, or a light petroleum fraction having a rather narrow boiling range ispreferred. For reservoirs at low temperatures and pressures, a solvent having an average molecular weight slightly lower than heptane may be preferred. For reservoirs at high temperatures and pressures, a solvent with an average molecular weight somewhat higher than heptane can be used. The solvent may have an average molecular weight as high as kerosene or even higher in some cases. The hydrocarbon solvent for the polymer must of course, have a molecular weight higher than that of propane.

Various types of LPG can be obtained having a composition for almost pure propane to almost pure butane. As previously mentioned, for our purposes, a type which is at least mostly propane should be used. Butane is a better polymer solvent than propane but the phase relationships with methane and heptane are not as good. Ethane has better phase relationships than propane but is a much worse polymer solvent. The differences between propane and ethane, on the one hand, and between propane and butane, on the other, are so great that propane is a rather unique material for our purposes in most formations. This does not mean, of course, that pure propane must be used. Even in the temperature range from about to F., and the pressure range from about 1,500 to 2,000 pounds per square inch, the propane may contain up to about 20 percent of ethane, butane, or mixtures of ethane and butane with traces of other hydrocarbons. At lower temperatures and pressures, a little more ethane, possibly up to almost 50 percent, may be used. At higher temperatures and pressures, up to almost 50 percent, may be used. At higher temperatures and pressures, up to almost 50 percent butane may be used with the propane. Thus, when theterm "propane" is used, it is intended that the material should be mostly propane, but may contain up to almost 50 percent of other hydrocarbons. Obviously, it is ordinarily advisable to select some inexpensive commercial LPG containing a mixture of propane with other hydrocarbons as the principal displacing fluid for the petroleum.

The LPG used in the polymer solution and in the enriched gas may be somewhat different, but preferably the two are at least about the same in both cases.

Methane has several possible alternates as the dry gas and the base for the enriched gas. These include air, nitrogen, flue gas, and the like. Methane, particularly natural gas, is greatly preferred, however. The ordinary driving gas, which displaces the enriched gas through the formation, is frequently referred to as a dry gas. it may, however, be a dry gas only in the sense that it has not been enriched. it may contain naturally present ethane, propane, and the like.

Our invention will be better understood from the following example. An oil-bearing formation has a temperature of 100 F. and is at a pressure of about 1,500 pounds per square inch absolute. This formation is penetrated by several wells. Available in the area is a condensate having an average molecular weight about the same as heptane. Also available is propane containing 4 percent butane and traces of ethane. A source of dry natural methane gas is also available. A polyisobutylene polymer is obtained having a molecular weight of about 130,000, as measured by the Staudinger specific viscosity method. it is found that a solution containing 0.25 weight percent polymer in a solvent which is mostly propane with less of the condensate has a viscosity about 0.2 times the viscosity of the oil to be displaced. The maximum amount of propane which will hold this amount of polymer in solution at surface conditions is found to be about 75 percent by volume. The polymer solvent, therefore, should contain about 75 percent propane and about 25 percent condensate.

The pore volume of the reservoir expected to be flooded is a little over a million barrels (42 US. gallons per barrel). it is decided that the polymer solution bank should be about 5 percent of this volume or 50,000 barrels. About 30,000 pounds of polymer are dissolved in 12,500 barrels of condensate to form about a 1 percent by weight solution. This is done in batches. The solution of polymer in condensate is then diluted under s z pressure with about 37,500 barrels of propane. The result is 50,000 barrels of a polymer solution-in condensate and propane. This solution is injected into the. oil-bearing formation through injection wells. An enriched gas is then prepared containing about 44 percent propane and 56 percent methane. The volume of this gas is about 25,000barrels at formation conditions. The enriched gasis injected through the injection wells and into the formation following thepolymer solution. Dry natural gas is injected aftertheenriched gas to drive the oil, polymer solution and enriched gas through the formation to the producing wells. Oil is produced from the formation through the producing wells.

Many variations in our process are possible. For example, the polymer concentration may vary from a small fraction of 1 percent up to 2 percent, or even 3 percent, by weight of the total solution in the propane and heavier hydrocarbons. Ordinarily, however, the concentration should be less than 1 percent. This avoids any danger of plugging and also any danger of adverse phase effects by the po'rtionof the polymer which remains in solution upon dilution with enriched gas. The polymer concentration should also bekept low because of the high cost per pound of thepolymer compared to costs of condensate, propane, and the like. 7

Economic considerations also generally'require use of much more than 50 percent propane in the polymer solution. Under special circumstances, however, lower quantities of propane may-be used.

Water can be injected with both the polymer solution and the enriched gas. This not only causes the polymer solution and enriched gas to sweep through' a greater volume of the reservoir, but also reduces the amount of polymer solution and enriched gas required. I v

in the example, the volume of viscous propane was about percent of the reservoir pore volume expected to be flooded. The enriched gas volume was about-2.5 percent of this pore volume. These values can vary considerably depending principally upon the viscosity of the oil in the reservoir. A conservative estimate of volumes can be made .by calculating the amounts necessary to form the mixing zones as described in a paper entitled A Laboratory Study of Solvent Flooding, by- H.N. Hall and T. M. Geffen, in AIME Transactions, Volume 2l0, p. 48. in general, the volume of viscous propane will be about 2 percent to about 15 percent of the pore volume expected to be flooded. The enriched gas volume will be less than the propane volume and will be from about 1 percent to about 10 percent of the pore volume expected to be flooded.

These volumes are applicable to operations in which it is desired that a miscible drive continue from the injection well to the producing well. As little aslO or 20 barrels each of the viscous propane and enriched gas may be used to displace residual oil from the formation around a water-injection well in a regular waterflooding operation. In such cases, the drive loses its miscible character a few feet from the well, becoming a two-phase immiscible drive system.- Small treatments do have the advantage, however, of making the entire pore space near the injection well available for flow of injected water.

Still other variations will be apparent to those skilled in the art. Therefore, we do not wish to be limited by the descriptions and examples given above, but only by the following claims. r i

We claim; a

l. In a method for recovering oil froman underground oilbearing earth formation penetrated byaninjection well and a producing well, in which method propane is injected into said formation to displace oil toward said producing well from which said oil is produced to the surface, the improvement comprising injecting into said formation propane, the viscosity of which is increased by the presence-of a solid polymer and sufficient of a hydrocarbon solvent havinga molecular weight higher than that of propane to form a solution of said polymer in said propane, displacing the viscouspropane solution through the formation an enriched gas miscible with at least the liquid components of said viscous propane solution and finally displacing said enriched gas in the formation with dry gas.

2. The method of claim 1 in which said viscous propane solution is prepared by dissolving said polymer in said hydrocarbon solvent and then mixing propane with the solution of the polymer in the hydrocarbon solvent.

3. The method of claim 1 in -which said polymer is polyisobutylene.

4. The method of claim 1 in which said hydrocarbon solvent has an average molecular weight about the same as that of heptane.

5. The method of claim 1 in which said hydrocarbon solvent contains aromatic hydrocarbons.

I 6. The method of claim 1 in which said viscous propane solution has a viscosity between about 0.1 and about 0.2 times the viscosity of the oil in the formation.

7. The method of claim 1 in which the volume of said viscous propane solution is from about 2 percent to about 15 percent of the pore volume expected-to be flooded and the volume of said enriched gas is less than the volume of said viscous propane solution and is between about i percent and 7 about 10 percent of the pore volume expected to be flooded. 

2. The method of claim 1 in which said viscous propane solution is prepared by dissolving said polymer in said hydrocarbon solvent and then mixing propane with the solution of the polymer in the hydrocarbon solvent.
 3. The method of claim 1 in which said polymer is polyisobutylene.
 4. The method of claim 1 in which said hydrocarbon solvent has an average molecular weight about the same as that of heptane.
 5. The method of claim 1 in which said hydrocarbon solvent contains aromatic hydrocarbons.
 6. The method of claim 1 in which said viscous propane solution has a viscosity between about 0.1 and about 0.2 times the viscosity of the oil in the formation.
 7. The method of claim 1 in which the volume of said viscous propane solution is from about 2 percent to about 15 percent of the pore volume expected to be flooded and the volume of said enriched gas is less than the volume of said viscous propane solution and is between about 1 percent and about 10 percent of the pore volume expected to be flooded. 